The present invention relates to a thermal analyzer for measuring the dependency of properties of a material upon the temperature. More specifically, the invention relates to a thermal analyzer for measuring the dependency of properties of a material upon the temperature by controlling the temperature according to a temperature program which elevates or lowers the temperature of a heating furnace for heating the sample at a constant rate.
In a conventional heat analyzer as disclosed in, for example,JP-A-1-174954, the heating furnace is maintained at a constant temperature prior to making the measurement to examine the relationship between the sample/heating furnace temperature deviation and the sample temperature. When really making the measurement, the temperature deviation is added to the programmed temperature, and thus the temperature of the heating furnace is controlled to be somewhat higher than its programmed temperature, so that the sample temperature is controlled to approach its programmed temperature.
In a feedback loop for controlling the temperature of the heating furnace, further, the temperature is controlled so as to bring the sample temperature close to its programmed temperature by using the sample temperature as the feedback quantity instead of using the heating furnace temperature.
In the heat analyzer, the heating furnace is controlled by feedback such as PID control so that the temperature of the heating furnace is brought into agreement with the programmed temperature. Therefore, the temperature of the heating furnace is so controlled as to follow the programmed temperature at all times. Between the heating furnace and the sample, however, there is loss due to heat resistance and time lag and, therefore, there is a temperature deviation at all times between the heating furnace and the sample. This will be no problem if the temperature deviation is constant. In practice, however, the temperature deviation varies depending upon the temperature of the heating furnace and the rate of elevating or lowering the temperature. In general, the deviation increases with increase in the temperature of the heating furnace and with an increase in the rate of elevating or lowering the temperature.
FIGS. 3 and 5 illustrate examples of measuring a relationship between the temperature of the heating furnace and the sample temperature. FIG. 3 illustrates a case of when the temperature is elevated and FIG. 5 illustrates a case of when the temperature is lowered. The temperature deviation between the heating furnace and the sample increases with an increase in the temperature zone or with an increase in the rate of elevating the temperature (lowering the temperature). The same data are also used in FIGS. 4 and 6 where the X-axis represents the sample temperature and the Y-axis represents the deviation of temperature between the heating furnace and the sample. The temperature deviation increases nearly in proportion to the rate of elevating the temperature (lowering the temperature). FIG. 4 includes additional approximation curves of temperature deviations held constant at each of the temperatures, as a curve of 0° C./min. When held constant (rate of temperature elevation is 0) as described above, the temperature deviation becomes still smaller.
In the former case according to the above prior art, the temperature deviations are examined in a state where the temperatures are maintained constant, and the relationship between the temperature of the heating surface and the sample temperature is polynomially approximated so thatTemperature of the heating furnace=f(sample temperature)and the correction is effected by using the approximation formula f at the time of making a measurement. In the case of the temperature program maintaining a constant temperature, the heating furnace and the sample temperature are maintained to be in good agreement as desired.
However, the temperature program which elevates or lowers the temperature at a constant rate has the problem of increased temperature deviation between the heating furnace and the sample. In particular, the temperature deviation increases with an increase in the rate of elevating the temperature or in the rate of lowering the temperature.
Further, the latter case has the problem that it is difficult to execute stable control due to the time lag in the change of the feedback value (sample temperature) which indicates the temperature of the heating furnace which is the object to be controlled.
It is the problem of the present invention to provide a thermal analyzer which enables the temperature of a sample to accurately follow the programmed temperature even in a temperature program which elevates or lowers the temperature at a constant rate.